O......K........ well I already gave you three to start with, but if you really want i'll give you another five, if it means that much to you. Although I think that i'll do them as answers to your 'good' points
You say - Grittier, more involving story.
I Say - There was no advance in 'grittiness'. CJ returns from liberty city where he's been making a living by robbing people, because his mum is killed by baller's doing a drive-by. Upon his return he is forced down a path of criminality by three corrupt policemen, two of whom are covering up their murder of a fellow policeman. To me as a story, that is pretty damn gritty, and tbh i found the story of GTA4 the least involving of the series, including GTA: Vice City Stories, the story of which was terrible
You Say - More believable characters.
I Say - Really? how many ex people smugglers do you know who went on to live illegally in a major metropolitan area commiting atrocious crimes and getting away with it? The trick that rockstar (and seemingly you) have missed is that the actions you are required to commit in a game such as GTA require an exaggerated arena in which to take place. Trying to ramp up the realism in a manufactured locale is fine, but it really jars with the immersion of the experience if the actions of your protagonist defies the verisimilitude of the setting.
You Say - Much more well designed and thought out city.
I Say - No chance! unless you enjoy queueing to pay to cross a bridge, or if you do decide to take a girlfriend out having to either be in the general location before making the phone call, or get a taxi to her place unless you like arriving at her place just as the notification appears to tell you that you are too late. The city design was o.k. sure, but not a selling point and certainly not the leap in level design that the original liberty city offered. You also have to analyze the validity of the city layout, being as the missions were basically point to point, and the sandbox mode upon completion of the main adventure was basically useless.
You Say - Greater levels of realism in vehicle handling and combat (especially CQC).
I Say - First off i have no idea what cqc is, but in any review the vehicle handling has been lambasted, and there is a reason for this - they have ramped up the realism! There is a fine line between believability and fun in games, and rockstar have crossed that line with the vehicles. If you find these adjustments to the game mechanics improvements, then why dont we go the whole hog and have car keys, re-fueling at petrol stations and having to renew your tax disc? As for the combat, it has been vastly improved, especially the cover system which should really have been implemented long ago.
You Say - More accessible HUD system w/directions to waypoints.
I Say - I didn't really find the last hud system that inaccessable, tbh. Also the introduction of the sat nav in every car helped, sure, but i think its also contributed to the seeming linearity that has encroached on the series.
Thankyou for reading, i really don't want to come across as sacastic but these are my viewpoints.